Receiver for an LDPC based TDS-OFDM communication system

ABSTRACT

An LDPC based TDS-OFDM receiver for demodulating an LDPC encoded TDS-OFDM modulated RF signal downconverted to an IF signal includes a synchronization block, an equalization block, an OFDM demodulation block and a FEC decoder block. The synchronization block generates a baseband signal from a digitized IF signal and performs correlation of a PN sequence in a signal frame of the received RF signal with a corresponding locally generated PN sequence to provide signals for performing carrier recovery, timing recovery and parameters for channel estimation. The equalization block performs channel estimation and channel equalization. The OFDM demodulation block performs demodulation on the baseband signal to recover OFDM symbols and converts the OFDM symbols to frequency domain. The FEC decoder block includes an LDPC decoder for decoding the OFDM symbols based on the LDPC code to generate a digital output signal indicative of the data content of the RF signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/820,319, filed on Jul. 25, 2006, whichapplication is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to an LDPC based TDS-OFDM communication systemand, in particular, to an LDPC based TDS-OFDM receiver incorporating anLDPC decoder and an OFDM demodulator for use in the LDPC based TDS-OFDMcommunication system.

DESCRIPTION OF THE RELATED ART

Transmission of Internet signals and of digital television (TV) signalsposes different but continuing challenges for each activity. Internetsignal transmission faces the problems of reliable broadcasting andmulticasting of messages, provision of mobility for signal transmitterand for recipient, and limitations on information transfer rate(“speed”). Transmission of digital TV signals faces the problems ofproviding an inter active system, providing point-to-point informationtransfer capacity, and mobility of the recipient. A communication systemfor digital TV signals should be efficient in the sense that the payloador data portion of each transmitted frame is a large fraction of thetotal frame. At the same time, such a communication system should beable to identify, and compensate for, varying characteristics of thetransmission channel, including but not limited to time delay associatedwith transmission of each frame.

Orthogonal frequency-division multiplexing (“OFDM”) modulation schemesare known. OFDM, also sometimes referred to as discrete multitonemodulation (DMT), is a complex modulation technique for transmissionbased upon the idea of frequency-division multiplexing (FDM) where eachfrequency channel is modulated with a simpler modulation. In OFDM, thefrequencies and modulation of FDM are arranged to be orthogonal witheach other which almost eliminates the interference between channels.Conventional OFDM transmission uses a guard interval period between OFDMdata frames to provide a protective separation between transmitted OFDMframes. The guard interval is usually only necessary when multipatheffect is of concern. The guard interval can include zero-padding or acyclic prefix of the OFDM data. The guard interval represents a timeperiod between OFDM data frames effective for protecting data againstinter-subcarrier interference in the frequency domain and inter-frameinterference in the time domain.

U.S. Pat. No. 7,072,289 to Lin Yang et al. describes a time-domainsynchronous (TDS) OFDM modulation scheme (TDS-OFDM). In TDS-OFDMmodulation, a pseudo-noise (PN) sequence is inserted in the protectiveguard interval between data frames to enable synchronization and channelestimation. The TDS-OFDM modulation is different from the conventionalOFDM or coded OFDM modulation in that the TDS-OFDM modulation schemedoes not use the cyclic repeat of the OFDM data in the guard interval.Rather, a PN sequence is used to allow the receiver to achieve fastersynchronization, faster frame and timing recovery and a more robustchannel estimation to recover the transmitted information with noadditional loss of spectrum efficiency. In one application, a TDS-OFDMmodulation scheme utilizes 3780 symbols representing a fixed FFT (FastFourier Transform) size of 3780. The fixed FFT size is particularlyuseful for the TDS-OFDM modulation scheme. In general, TDS-OFDMmodulation schemes can use a FFT size of 2^(n) integer multiple and theFFT size does not have to be fixed.

Error correction and channel coding schemes are often employed to reducetransmission errors in a communication channel. A low-density paritycheck (LDPC) code is an error correcting code for use in transmitting amessage over a noisy transmission channel and performing forward errorcorrection (FEC). An LDPC code may be viewed as being a code having abinary parity check matrix such that nearly all of the elements of theparity check matrix have values of zeros. While LDPC codes and othererror correcting codes cannot guarantee perfect transmission, theprobability of lost information can be made as small as desired. LDPCcodes were the first coding scheme to allow data transmission ratesclose to the theoretical maximum, the Shannon Limit. At its discovery,LDPC codes were found to be impractical to implement in most cases dueto the heavy computational burden in implementing the codes and theencoder for the codes and thus were not widely used. However, LDPC codeshave since been rediscovered and have been widely applied incommunication systems.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, an LDPC basedTDS-OFDM receiver for receiving and demodulating an RF signal modulatedusing a TDS-OFDM (Time-Domain Synchronous Orthogonal Frequency-DivisionMultiplexing) modulation scheme and encoded using an LDPC (low densityparity check) code where the RF signal is downconverted to an IF signalincludes a synchronization block, an equalization block, an OFDMdemodulation block and a FEC decoder block. The synchronization blockreceives a digital IF signal indicative of the IF signal and generates abaseband signal from the digital IF signal. The synchronization blockperforms correlation of a PN sequence in a signal frame of the receivedRF signal with a corresponding locally generated PN sequence where thePN sequence correlation providing signals for performing carrierrecovery, timing recovery and parameters for channel estimation. Theequalization block performs channel estimation and channel equalization.The OFDM demodulation block performs demodulation on the baseband signalwhere the OFDM demodulation block recovers OFDM symbols in the signalframe of the RF signal and converts the OFDM symbols to frequencydomain. The FEC (forward error correction) decoder block includes anLDPC decoder for decoding the OFDM symbols based on the LDPC (lowdensity parity check) code to generate a digital output signalindicative of the data content of the RF signal.

According to another aspect of the present invention, a method fordemodulating an LDPC encoded TDS-OFDM modulated RF signal includesreceiving a digital IF signal being down-converted from a RF signal;converting the digital IF signal to a baseband signal; performingsynchronization of the baseband signal using the PN sequence in thesignal frame of the RF signal; generating signals for carrier recovery,timing recovery and channel estimation; performing channel estimationusing the signals for channel estimation; recovering OFDM symbols fromthe baseband signals; performing equalization of the baseband signal;performing FEC decoding of the OFDM symbols, the FEC decoding comprisingdecoding the OFDM symbols based on an LDPC (low density parity check)code; and generating a digital output signal indicative of the datacontent of the RF signal.

The present invention is better understood upon consideration of thedetailed description below and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an LDPC based TDS-OFDM receiver accordingto one embodiment of the present invention.

FIG. 2 illustrates a frame format for a TDS-OFDM signal frame accordingto one embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating the functional blocks of anLDPC based TDS-OFDM receiver according to one embodiment of the presentinvention.

FIGS. 4A and 4B include a flow chart illustrating the data flow of thereceived RF signals through the LDPC-based TDS-OFDM receiver accordingto one embodiment of the present invention.

FIGS. 5( a)-(c) illustrate the signal frame structure for signal frameswith PN420, PN595 and PN945 guard intervals, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the principles of the present invention, an LDPCbased TDS-OFDM receiver for use in an LDPC based TDS-OFDM communicationsystem includes a synchronization block, an OFDM demodulator and an LDPCdecoder where the synchronization of the received signal is based on thePN sequence inserted in the guard intervals of the signal frames. Thereceiver receives LDPC encoded TDS-OFDM modulated incoming RF (radiofrequency) signals and performs TDS-OFDM demodulation to recover theOFDM symbols in the received RF signals. Finally, LDPC forward errorcorrection (FEC) decoding is performed to recover the MPEG-2 transportstream in the received RF signals. The MPEG-2 transport stream includesvalid data, synchronization and clock signals. The LDPC based TDS-OFDMreceiver of the present invention can work in either single frequency ormultiple frequency networks.

The LDPC based TDS-OFDM communication system provides many advantagesover conventional communication systems and is particularly advantageouswhen applied in digital television broadcast systems to ensure enhancedtelevision reception. One salient characteristic of the LDPC basedTDS-OFDM communication system of the present information is that in eachsignal frame, the number of MPEG-2 transport stream data packets isalways a pre-determined integer (e.g.: 2, 3, 4, 6, 8, 9, 12). Inconventional OFDM modulation systems, the deterministic number of datapackets in each signal frame is often not possible under allcircumstances.

In the present description, a TDS-OFDM communication system refers to acommunication system that utilizes the TDS-OFDM modulation schemedescribed in the aforementioned U.S. Pat. No. 7,072,289, entitled“Pseudo-Random Sequence Padding In An OFDM Modulation System,” issued toLin Yang et al., which patent is incorporated herein by reference in itsentirety. In TDS-OFDM modulation, a PN (pseudo-noise) sequence is usedin the data block guard intervals where the PN sequence satisfiesselected orthogonality and closures relations. That is, the guardinterval between each data frame is a PN sequence of a given length. ThePN sequence is used for timing recovery, for carrier frequency recovery,channel estimation and synchronization. The PN sequence enables thereceiver of the TDS-OFDM communication system to achieve fastersynchronization, faster frame and timing recovery and a more robustchannel estimation.

Furthermore, in the present description, the LDPC based forward errorcorrection is implemented using a set of three LDPC codes havingcharacteristics that are compatible with and particularly advantageousfor application in a TDS-OFDM communication system. In one embodiment,the RF signals received by the LDPC based TDS-OFDM receiver of thepresent invention are transmitted under the TDS-OFDM transmission schemedescribed in copending and commonly assigned U.S. patent applicationSer. No. 11/691,102, entitled “Transmitter For An LDPC based TDS-OFDMCommunication System,” of D. Venkatachalam et al., filed on Mar. 26,2007, which patent application is incorporated herein by reference inits entirety. In particular, the TDS-OFDM transmission scheme uses 3780symbols and the parameters of the LDPC codes are tuned to the 3780symbol-based TDS-OFDM transmission scheme. The use of LDPC based forwarderror correction in the TDS-OFDM communication system of the presentinvention enables superior error correction properties approaching theShannon Limit of the channel.

In one embodiment, the LDPC based TDS-OFDM receiver of the presentinvention is configured to decode the three quasi-cyclic LDPC codesdescribed in copending and commonly assigned U.S. patent applicationSer. No. 11/685,539, entitled “LDPC Codes Of Various Rates For An LDPCbased TDS-OFDM Communication System,” by Lei Chen, filed on Mar. 13,2007, which patent application is incorporated herein by reference inits entirety. In the '539 patent application, three quasi-cyclic LDPCcodes of rate 0.4, 0.6 and 0.8 and their associated parity checkmatrices are described. The three LDPC codes of the '539 patentapplication is implemented in the LDPC based TDS-OFDM communicationsystem of the present invention to achieve superior error correctionproperties approaching the Shannon Limit of the channel.

FIG. 1 is a block diagram of an LDPC based TDS-OFDM receiver accordingto one embodiment of the present invention. Referring to FIG. 1, an LDPCbased TDS-OFDM receiver 10 (“receiver 10”) is coupled to receiveincoming RF signals received on an antenna 12 and generate MPEG-2transport stream as output signals. The primary function of receiver 10is to determine from a noise-perturbed system, which of the finite setof waveforms have been sent by the transmitter and using a combinationof signal processing techniques to reproduce the finite set of discretemessages sent by the transmitter.

In the present embodiment, the incoming RF signals are received by an RFtuner 14 where the RF input signals are converted to low-IF or zero-IFsignals. In one embodiment, a digital terrestrial tuner is used toreceive the incoming RF signal and picks the frequency bandwidth ofchoice to be demodulated by receiver 10. From tuner 14, the low-IF orzero-IF signals are provided to receiver 10 as analog signals or asdigital signals. For instance, an optional analog-to-digital converter60 may be coupled to tuner 14 to digitize the down-converted IF signalsso that digitized down-converted IF signals are provided to receiver 10without further digitizing. In the present embodiment, as shown in FIG.1, an analog-to-digital converter 16 is provided in receiver 10 todigitize the analog IF signals from tuner 14.

In one embodiment, the LDPC based TDS-OFDM receiver 10 is implemented asa single integrated circuit. The tuner 14 is implemented outside of theintegrated circuit of receiver 10. In other embodiments, the LDPC basedTDS-OFDM receiver may incorporate the tuner circuitry on the sameintegrated circuit. The degree of integration of receiver 10 is notcritical to the practice of the present invention.

The LDPC based TDS-OFDM receiver 10 of the present invention includesfour major functional blocks: a synchronization block 18, anequalization block 20, an OFDM demodulation block 22 and an LDPC FECdecoder block 24. In receiver 10, the synchronization block 18 operateson the PN sequence inserted in the guard intervals of the signal framesand performs Auto Frequency Control (AFC), carrier recovery to determinethe carrier frequency, and timing recovery to determine the timing ofthe incoming signals. Synchronization block 18 also performs framesynchronization (FSYNC) to determine the start of each signal frame inthe incoming signals and the frame ID of each signal frame so that thePN sequence associated with each signal frame used in the TDS-OFDMmodulation can be determined. The equalization block 20 performs channelestimation and channel equalization. For OFDM modulation, the channelequalizer is typically a divider or a single multiplier.

OFDM demodulator 22 performs demodulation of the received signals byextracting the symbols and converting the signals to frequency domain.The LDPC FEC decoder 24 then provides forward error correction of thereceived signals based on the LDPC codes used to encode the datamessage. In the present embodiment, LDPC FEC decoder 24 decodes thereceived signals in accordance with the selected one of the threequasi-cyclic LDPC codes described in the aforementioned '539 patentapplication. Finally, receiver 10 generates an MPEG-2 transport streamas output signals indicative of the received RF signals.

The structure of the incoming data signals is first described. Thedigital data signals under the LDPC based TDS-OFDM transmission schemeare grouped in a series of hierarchical frames. FIG. 2 illustrates aframe format for a TDS-OFDM signal frame according to one embodiment ofthe present invention. Referring to FIG. 2, a TDS-OFDM signal frameincludes a data frame of 3780 symbols preceded by a guard interval ofvarious length. The data frame includes 3744 symbols for carrying thedata payload and 36 symbols for carrying the Transmission ParameterSignaling (TPS). The TPS symbols carry information for the demodulatorof the receiver to automatically adapt to the incoming transmission suchas the FEC inner code rate and the time interleaver length.

In one embodiment, the length of the guard interval can either be theframe length (3780 symbols) divided by 9 (420 symbols), as shown in FIG.5( a), or the frame length divided by 4 (945 symbols), as shown in FIG.5( c). The length of the guard interval can also have a length of 595symbols, as shown in FIG. 5( b).

Turning now to the detailed construction of the LDPC based TDS-OFDMreceiver of the present invention. FIG. 3 is a schematic diagramillustrating the functional blocks of an LDPC based TDS-OFDM receiveraccording to one embodiment of the present invention. Referring to FIG.3, an LDPC based TDS-OFDM receiver 100 (hereinafter “receiver 100”)includes functional blocks implementing the synchronization,equalization, LDPC FEC decoding and OFDM demodulation functions wherethe TDS-OFDM synchronization and demodulation are performed according tothe parameters of the LDPC based TDS-OFDM transmission scheme.

The configuration parameters of receiver 100 can be detected, orautomatically programmed, or manually set. The main configurableparameters for receiver 100 include: (1) Subcarrier modulation type:QPSK, 16QAM, 64QAM; (2) FEC rate: 0.4, 0.6 and 0.8; (3) Guard interval:420, 595 or 945 symbols; (4) Time deinterleaver mode: 0, 240 or 720symbols; (5) Control frames detection; and (6) Channel bandwidth: 6, 7,or 8 MHz. The output signals of receiver 100 consist a parallel orserial MPEG-2 transport stream including valid data, synchronization andclock signals.

Turning to FIG. 3, the incoming RF signals received on an antenna 12 arecoupled to a tuner 14 to be down-converted to low-IF or zero-IF signals.In the present embodiment, the analog low-IF or zero IF signals areprovided to receiver 100 where an analog-to-digital converter (ADC) 102in receiver 100 samples and digitizes the low-IF/zero-IF signals. Inother embodiments, an ADC 60 may be provided outside of receiver 100 todigitize the down-converted IF signals and receiver 100 receives digitallow-IF or zero-IF signals, as shown by the dotted line from ADC 60. Inthis case, ADC 102 is not needed and the digitized IF signals arecoupled directly to the respective blocks of the receiver 100.

In LDPC based TDS-OFDM receiver 100, the digitized IF signals areprovided to the automatic gain control (AGC) block 104 and theIF-to-Baseband conversion block 106. AGC block 104 compares thedigitized IF signal strength with a reference. The difference isfiltered and the filtered comparison result is fed back to tuner 14 tocontrol the gain of the IF signals. More specifically, the filteredcomparison result is used to control the gain of an amplifier in tuner14.

At the IF-to-Baseband conversion block 106, the digitized IF signalsfrom ADC 102 are converted to baseband signals. When the analog IFsignals are sampled by ADC 102, the resulting digital signals arecentered at a lower intermediate frequency (IF). For example, sampling a36 MHz IF signal at 30.4 MHz results in the signal centered at afrequency of 5.6 MHz. The IF-to-Baseband conversion block 106 convertsthe digitized lower IF signal to a complex signal in the basebandfrequency.

The baseband frequency signals are then provided to a sample rateconverter 108. When the analog IF signals are digitized by ADC 102, theanalog-to-digital converter uses a fixed sampling rate. Sample rateconverter 108 operates to convert the signals from the fixed samplingrate used by the ADC to the OFDM sample rate. In one embodiment, samplerate converter 108 includes an interpolator to implement the sample rateconversion.

Sample rate converter 108 receives operation parameters from a timingrecovery block 110. Timing recovery block 108 operates based on theTDS-OFDM modulation scheme where the PN sequence inserted in the guardintervals of the data frames is used to provide time recoveryinformation. Timing recover block 108 receives input signals from a PNsequence correlation block 116 and computes the timing error. Timingrecovery block 108 filters the error and use the filtered error signalto drive an NCO (Numerically Controlled Oscillator). The NCO controlsthe sample timing correction to be applied in the interpolator of thesample rate converter 108.

In accordance with the LDPC based TDS-OFDM transmission scheme of thepresent invention, the transmitted signal frame is filtered using aSquare Root Raised Cosine (SRRC) filter. The received signals aretherefore also applied with the same filter function in the shapingblock 112. Shaping block 112 filters the converted signals by a SquareRoot Raised Cosine filter.

The output signal of shaping block 112 is provided to an automaticfrequency control (AFC) block 114. There are often frequency offsets inthe incoming RF signals. AFC block 114 calculates the frequency offsetsand adjusts the IF-to-Baseband reference IF frequency for theIF-to-Baseband conversion block 108. In one embodiment, the frequencycontrol under AFC block 114 is carried out in a coarse stage and a finestage to improve the capture range and tracking performance.

The output signals from shaping block 112 are coupled to the PN sequencecorrelation block 116. PN sequence correlation block 116 is specific tothe TDS-OFDM modulation scheme where a PN sequence is inserted into theguard intervals of the data frames. The correlation block 116 performsframe synchronization to determine the frame ID of each signal frame.From the frame ID, the PN sequence associated with each signal frame isdetermined. Then, the incoming PN sequence is correlated with thelocally generated PN sequence to find the correlation peak whichindicates the start of each signal frame and other synchronizationinformation such as frequency offset and timing error. The retrievedtiming error information is provided to timing recovery block 110 whilethe frequency offset information is provided to the AFC block 114 tofacilitation the processing of the incoming IF signals, as describedabove.

At this point, receiver 100 has completed the synchronization functionand the equalization function begins. The signals are provided to achannel estimation block 118. The channel time domain response isestimated based on the signal correlation obtained by PN sequencecorrelation 116. The channel frequency response is obtained by takingthe Fast Fourier Transform (FFT) of the time domain response. In thepresent embodiment, a TDS-OFDM modulation scheme utilizes 3780 symbolsrepresenting a fixed FFT (Fast Fourier Transform) size of 3780. In otherembodiments, TDS-OFDM modulation schemes can use a FFT size of 2^(n)integer multiple and the FFT size does not have to be fixed.

After the channel estimation block 18, the OFDM symbols in the signalsare recovered and restored. In TDS-OFDM modulation, a PN sequencereplaces the traditional cyclic prefix in the guard intervals. The OFDMsymbol restoration block 120 operates to remove the PN sequence from theguard interval and restore the channel spread OFDM symbols. The OFDMsymbol restoration block 120 essentially reconstructs the conventionalOFDM symbols which can then be one-tap equalized.

After the OFDM symbols are recovered, the symbols are provided to a FFTblock 122 to perform a Fast Fourier Transform operation. In the presentembodiment, the FFT block 122 performs a 3780 point FFT. In the presentembodiment, OFDM demodulator can operate in a multi-carrier mode or asingle-carrier mode. In the multi-carrier mode, the recovered symbolsare data in the time domain and the symbols are passed to the FFT block122 where the FFT operation converts the symbols into correspondingsignals in the frequency domain. On the other hand, in single-carriermode, the recovered symbols are directly presumed to be the data infrequency domain. Therefore, when the single-carrier mode is selected,the FFT operation is bypassed.

Next, at block 124, channel equalization is carried out on the FFTtransformed data based on the frequency response of the channel.Derotated data and the channel state information are sent to forwarderror correction for further processing. The output of channelequalization block 124 is sent to a time deinterleaver 126. In receiver100, time deinterleaver 126 is used to increase the resilience of thereceived signals to spurious noise. The time deinterleaver 126 is aconvolutional deinterleaver which needs a memory 65 with sizeB*(B−1)*M/2, where B is the number of the branch, and M is the depth. Inthe present embodiment, time deinterleaver 126 operates in one or twomodes. For mode 1, B=52 and M=240, while for mode 2, B=52, M=720. In thepresent embodiment, on-chip memory 65 is provided for time deinterleaver126. In other embodiments, a memory 66 external to receiver 100 can alsobe used.

After the time deinterleaver, the symbols are provided to forward errorcorrection. First, inner FEC is performed. The inner FEC is an LDPCdecoder 128 which is a soft-decision iterative decoder for decoding theQuasi-Cyclic Low Density Parity Check (QC-LDPC) code provided by thetransmitter. In the present embodiment, the LDPC decoder 128 isconfigured to decode 3 different rates (rate 0.4, rate 0.6 and rate 0.8)of QC-LDPC codes while using the same circuitry. The iteration processis either stopped when it reaches the specified maximum iteration number(full iteration), or when the detected error is free during errordetecting and correcting process (partial iteration).

The data are then passed to the outer FEC which is a BCH decoder 130.The BCH decoder 130 is constructed to decode BCH (762, 752) code, whichis the shortened binary BCH code of BCH (1023, 1013). The generatorpolynomial is 1+x¹⁰+x³.

In the present embodiment, the TDS-OFDM communication system is amulti-rate system based on multiple modulation schemes (QPSK, 16QAM,64QAM), and multiple coding rates (0.4, 0.6, and 0.8), where QPSK standsfor Quad Phase Shift Keying and QAM stands for Quadrature AmplitudeModulation. The output of BCH decoder 130 is a bit-by-bit data signal.Based on the selected modulation scheme and coding rate, a rateconversion block 132 operates to combine the bit output of BCH decoder130 into bytes, and adjust the speed of byte output clock so that theMPEG packets outputted by receiver 100 are evenly distributed during thewhole demodulation process.

Finally, under the TDS-OFDM transmissions scheme, the transmitted datais randomized using a pseudo-random (PN) sequence before the BCHencoder. Thus, the error corrected data by the LDPC/BCH decoder 128/130must then be de-randomized or descrambled. Descrambler 134 performs thedescrambling using a PN sequence generated by the polynomial 1+x¹⁴+x¹⁵,with initial condition of 100101010000000. Descrambler 134 will be resetto the initial condition for every signal frame. Otherwise, thedescrambler will be free running until it is reset again. The leastsignificant 8-bit of the PN sequence is XORed with the input bytestream.

The data flow through the LDPC based TDS-OFDM receiver 100 is nowdescribed with reference to the flow chart of FIGS. 4A and 4B. Referringto FIG. 4A, a LDPC based TDS-OFDM demodulation method 300 starts byreceiving the down-converted IF signal indicative of an LDPC encodedTDS-OFDM modulator RF signal (step 302). For instance, the RF signal canbe received by a tuner which picks the frequency bandwidth of choice tobe demodulated and then down-converts the RF signal to a baseband or lowintermediate frequency (IF) signal. The downconverted IF signal is thenconverted to the digital domain through analog-to-digital conversion(step 304). The analog-to-digital conversion can be part of thedemodulation method 300 or the ADC can be performed else where and thedigitized IF signal being provided to the demodulation method 300. Thedigitized IF signal is then converted to a baseband signal (step 306).The IF-to-Baseband conversion is carried out using frequency offset date307 obtained from an automatic frequency control operation.

The baseband signal is subjected to sample rate conversion (step 308).The sample rate conversion is carried out using timing error data 309generated by the subsequent PN sequence correlation process (step 312)where the timing error data is fed back to the sample rate conversionstep. After sample rate conversion, the signal is filtered using aSquare Root Raised Cosine filter for shaping (step 310). The signal isthen converted to symbols.

Then at step 312, the PN sequence information inserted in the guardinterval of the signal frames is extracted and correlated with a localPN generator to find the time domain impulse response. The FFT of thetime domain impulse response gives the estimated channel response (step314). The PN sequence correlation is also used for the timing recoveryto generate the timing error data 309 and the frequency estimation togenerate the frequency offset data 307. The timing error and frequencyoffset data are used to correct the received signal.

The OFDM symbol information in the received data is then extracted (step316). Referring now to FIG. 4B, in the multi-carrier mode, the OFDMsymbols are passed through a 3780 FFT (step 318) to convert the symbolinformation back in the frequency domain. Using the channel estimationinformation obtained at step 314, the OFDM symbols are equalized (step320) and the demodulation process continues.

When multi-carrier mode is selected, the symbols are subjected to a FastFourier Transform (FFT) operation (step 318). In the present embodiment,the FFT is a 3780-point FFT. The FFT converts the time domaininformation into a frequency domain signal. In some applications, asingle-carrier mode is selected. In that case, the FFT operation (step318) is bypassed as shown in FIG. 4B.

After the symbols are equalized (step 320), a time deinterleavingoperation (step 322) is performed where the transmitted symbol sequenceis subject to deconvolution. Then the 3780 blocks of symbol sequence ispassed to the inner FEC decoder for LDPC decoding (step 324). The LDPCdecoding is based on three LDPC codes of various rates (325) used toencode the transmitted signal. Then, the symbol sequence is passed tothe outer FEC decoder for BCH decoding (step 326). The LDPC and BCHdecoding operations run in a serial manner to take in exactly 3780symbols, remove the 36 TPS symbols and process the remaining 3744symbols. The LDPC and BCH decoding operations recover the transmittedtransport stream information from the symbol sequence. Then, rateconversion (step 328) is performed to adjust the output data rate.Finally, the transport stream information is descrambled (step 330) toreconstruct the transmitted MPEG-2 transport stream information (step332).

The above detailed descriptions are provided to illustrate specificembodiments of the present invention and are not intended to belimiting. Numerous modifications and variations within the scope of thepresent invention are possible. For example, in the present embodiment,the receiver generates MPEG-2 transport stream as the output signal. Theuse of MPEG-2 transport stream is illustrative only. The LDPC basedTDS-OFDM receiver can generate digital output signals containing video,audio and data in any format desired by the user. The present inventionis defined by the appended claims.

1. An LDPC based TDS-OFDM receiver for receiving and demodulating an RFsignal modulated using a TDS-OFDM (Time-Domain Synchronous OrthogonalFrequency-Division Multiplexing) modulation scheme and encoded using anLDPC (low density parity check) code, the RF signal being downconvertedto an IF signal, the receiver comprising: a synchronization blockreceiving a digital IF signal indicative of the IF signal and generatinga baseband signal from the digital IF signal, the synchronization blockperforming correlation of a PN sequence in a signal frame of thereceived RF signal with a corresponding locally generated PN sequence,the PN sequence correlation providing signals for performing carrierrecovery, timing recovery and parameters for channel estimation; anequalization block for performing channel estimation and channelequalization; an OFDM demodulation block for performing demodulation onthe baseband signal, the OFDM demodulation block recovering OFDM symbolsin the signal frame of the RF signal and converting the OFDM symbols tofrequency domain; and a FEC (forward error correction) decoder blockcomprising an LDPC decoder for decoding the OFDM symbols based on theLDPC (low density parity check) code to generate a digital output signalindicative of the data content of the RF signal.
 2. The LDPC basedTDS-OFDM receiver of claim 1, wherein the IF signal comprises an analogIF signal and the receiver further comprises an analog-to-digitalconverter for digitizing the analog IF signal to the digital IF signal.3. The LDPC based TDS-OFDM receiver of claim 1, wherein the RF signal isdownconverted and digitized to the digital IF signal, the digital IFsignal being coupled to the synchronization block of the receiver. 4.The LDPC based TDS-OFDM receiver of claim 1, wherein the synchronizationblock comprises: an automatic frequency control block receiving thesignals from the PN sequence correlation and generating a frequencyoffset value; an IF-to-baseband conversion block for receiving thedigital IF signal and converting the digital IF signal to a basebandsignal, the IF-to-baseband conversion block receiving the frequencyoffset value; a timing recovery block receiving the signals from the PNsequence correlation and computing a timing error value; a sample rateconverter receiving the timing error and converting the sample rate ofthe baseband signal based on the timing error; a shaping block forapplying a filtering function to the baseband signal; and a PN sequencecorrelator for performing the PN sequence correlation.
 5. The LDPC basedTDS-OFDM receiver of claim 4, wherein the shaping block applies a SRRC(Square Root Raised Cosine) filter function to the baseband signal. 6.The LDPC based TDS-OFDM receiver of claim 1, wherein the equalizationblock and the OFDM demodulation block comprise: a channel estimationblock receiving the signals from the PN sequence correlation andestimating the channel time domain response and computing the channelfrequency response by taking the Fast Fourier Transform (FFT) of thetime domain response; an OFDM symbol restoration block for removing thePN sequence from the guard interval of the signal frame and recoveringthe OFDM symbols; a FFT block for perform a Fast Fourier Transform (FFT)operation on the OFDM symbols to convert the symbols to the frequencydomain; and a channel equalization block for performing equalization ofthe OFDM symbols based on the frequency response of the channel.
 7. TheLDPC based TDS-OFDM receiver of claim 6 wherein the RF signal comprisesa multi-carrier RF signal.
 8. The LDPC based TDS-OFDM receiver of claim6, wherein the FFT block performs a FFT operation using a fixed FFT sizeof
 3780. 9. The LDPC based TDS-OFDM receiver of claim 6 wherein the RFsignal comprises a single-carrier RF signal and the Fast FourierTransform operation is bypassed, the recovered OFDM symbols beingprovided directly to the channel equalization block.
 10. The LDPC basedTDS-OFDM receiver of claim 1, wherein the FEC block comprises: the LDPCdecoder for decoding the OFDM symbols based on the LDPC code used toencode the RF signal; and a BCH decoder for decoding the OFDM symbolsbased on a BCH code used to encode the RF signal.
 11. The LDPC basedTDS-OFDM receiver of claim 10, wherein the LDPC code comprises aquasi-cycle LDPC (QC-LDPC) code selected from a group of quasi-cycleLDPC codes having a rate selected from 0.4, 0.6 and 0.8.
 12. The LDPCbased TDS-OFDM receiver of claim 11, wherein the group of quasi-cycleLDPC codes comprises a (7493,3048) QC-LDPC code with a rate of 0.4, a(7493,4572) QC-LDPC code with a rate of 0.6, a (7493,6096) QC-LDPC codewith a rate of 0.8.
 13. The LDPC based TDS-OFDM receiver of claim 10,further comprising a time deinterleaver for performing a convolutionaldeinterleaving of the OFDM symbols before the FEC decoder.
 14. The LDPCbased TDS-OFDM receiver of claim 1, further comprising: a rateconversion block for combining the bit output of FEC decoder into bytes,and adjusting the speed of byte output clock to evenly distribute theoutput bytes; and a descrambler for descrambling the input data streamusing a pseudo-random (PN) sequence, thereby generating the digitaloutput signal indicative of the data content of the RF signal.
 15. TheLDPC based TDS-OFDM receiver of claim 1, wherein the digital outputsignal comprises a MPEG-2 transport stream.
 16. A method fordemodulating an LDPC encoded TDS-OFDM modulated RF signal, comprising:receiving a digital IF signal being down-converted from a RF signal;converting the digital IF signal to a baseband signal; performingsynchronization of the baseband signal using the PN sequence in thesignal frame of the RF signal; generating signals for carrier recovery,timing recovery and channel estimation; performing channel estimationusing the signals for channel estimation; recovering OFDM symbols fromthe baseband signals; performing equalization of the baseband signal;performing FEC decoding of the OFDM symbols, the FEC decoding comprisingdecoding the OFDM symbols based on an LDPC (low density parity check)code; and generating a digital output signal indicative of the datacontent of the RF signal.
 17. The method of claim 16, wherein performingsynchronization of the baseband signal comprises: performing acorrelation of a PN sequence in the signal frame of the received RFsignal with a corresponding locally generated PN sequence, the PNsequence correlation providing signals for determining the frequencyoffset, timing error and parameters for channel estimation; convertingthe digital IF signal to the baseband signal using the frequency offsetdata; converting the sample rate of the baseband signal using the timingerror data; and filtering the converted baseband signal using a filterfunction.
 18. The method of claim 17, wherein filtering the convertedbaseband signal comprises filtering the converted baseband signal usinga SRRC (Square Root Raised Cosine) filter.
 19. The method of claim 16,wherein the LDPC encoded TDS-OFDM modulated RF signal comprises amulti-carrier RF signal, the method further comprising: performing aFast Fourier Transform of the OFDM symbols before equalization.
 20. Themethod of claim 19, wherein performing a Fast Fourier Transform of theOFDM symbols before equalization comprises performing a Fast FourierTransform of the OFDM symbols before equalization using a fixed FFT sizeof
 3780. 21. The method of claim 16, wherein performing FEC decoding ofthe OFDM symbols comprises: decoding the OFDM symbols based on the LDPC(low density parity check) code; and decoding the OFDM symbols based ona BCH code.
 22. The method of claim 21, wherein the LDPC code comprisesa quasi-cycle LDPC code selected from a group of quasi-cycle LDPC(QC-LDPC) codes having a rate selected from 0.4, 0.6 and 0.8.
 23. Themethod of claim 22, wherein the group of quasi-cycle LDPC codescomprises a (7493,3048) QC-LDPC code with a rate of 0.4, a (7493,4572)QC-LDPC code with a rate of 0.6, a (7493,6096) QC-LDPC code with a rateof 0.8.
 24. The method of claim 16, wherein the LDPC encoded TDS-OFDMmodulated RF signal comprises a single-carrier RF signal.
 25. The methodof claim 16, further comprising: performing rate conversion of thedecoded OFDM symbols by combining the bit output of FEC decoding intobytes, and adjusting the speed of byte output clock to evenly distributethe output bytes; and descrambling the input data stream using apseudo-random (PN) sequence, thereby generating the digital outputsignal indicative of the data content of the RF signal.
 26. The methodof claim 16, wherein the digital output signal comprises a MPEG-2transport stream.
 27. The method of claim 16, wherein prior toperforming FEC decoding, performing time deinterleaving of the OFDMsymbols.